Turkish Prime Minister Recep Tayyip Erdogan lambasted the German media in a speech to around 15,000 supporters in western Germany on Saturday, crowning a week of mounting tension between the two countries.

Speaking to the packed crowd in a Cologne sports arena, Mr. Erdogan struck an uncompromising tone, saying German journalists had conspired with anti-government publications in Turkey to portray an inaccurate picture of the Soma...